If you have both Survival Remote Site Telephony (SRST) version 2.0 or
later and Media Gateway Control Protocol (MGCP) fallback configured on the
gateway, use this document in order to solve either of these MGCP failover
problems:

A telephone on a Foreign Exchange Station (FXS) port does not get a
dial tone, or you cannot make incoming calls through a Foreign Exchange Office
(FXO) port or through the ISDN link.

You cannot make incoming calls with direct inward dial (DID) to an
MGCP gateway, configured for MGCP fallback, in a failover
situation.

You are presented with a telephone on an FXS port that does not get a
dial tone, or you cannot make incoming calls through an FXO port or ISDN link.
In such situations, this is configured under the relevant plain old telephone
service (POTS) dial peer:

dial-peer voice X pots
application mgcpapp

Note: For Cisco IOS Software Release 12.3(7)T or later, the
application mgcpapp command must
not be applied to the POTS dial peer that supports PRI
Backhaul.

In this situation, an error message similar to this appears on the
console of the Cisco IOS gateway:

You cannot make incoming calls, with DID, to an MGCP gateway configured
for MGCP failover in a failover situation—even though there is a connection,
the phone does not ring. This is an example of the debug isdn
q931 command used on the Cisco IOS gateway, when you verify this
problem: